Silicate glasses, having a relatively high content of lead oxide in their compositions, have unique properties that particularly adapt them to use in production of crystal glassware. The properties of interest include a low liquidus temperature and a long viscosity range. These properties avoid the occurrence of devitrification during forming processes.
Environmental and health concerns have led to a desire to eliminate, insofar as possible, use of toxic oxides, in particular lead oxide, in glass compositions. U.S. Pat. No. 5,270,269 (Hares et al.) describes one approach to meeting this desirable end in the production of crystal glassware. The present invention provides a different approach.
The production of crystal glassware is usually a hand crafting operation. This involves several cycles of working and reheating a glass shape. It is, of course, desirable to complete as much of the shaping operation as possible during each cycle. This requires a low, liquidus temperature and a long, viscosity curve. The latter property translates into a long working range of temperatures without devitrification, that is, without any crystal formation in or on the glass.
Other properties of interest in crystal glassware include high refractive index, dispersion, and density values; also, good scratch resistance, and high chemical durability, that is, resistance to chemical attack and weathering. Another property of particular interest in thin-walled artware is the resonating sound that occurs when the rim of an article is lightly struck with another body such as a table utensil. This resonating sound is commonly known as the sonority, or ring, of a thin-walled article, such as a vase or goblet.
It is a purpose of the present invention to provide a family of glass compositions possessing properties useful both for crystal glassware and for laminated ware.
It is another purpose to provide a family of glass compositions free of toxic materials, in particular, free from lead compounds.
It is a further purpose to provide glasses having a CTE of 70-110.times.10.sup.-7 /.degree. C., a density of at least 2.70 g/cm.sup.3, a long viscosity curve, and a low liquidus value, such glasses being especially suited to production of crystal glassware.
It is a still further purpose to provide a family of glasses, as an alternative to the family disclosed in U.S. Pat. No. 5,270,269, for the production of crystal glassware.